In metal additive manufacturing, welding, and cladding applications, the quality of the powder stream is an important factor in build quality. It is important that the powder be consistently and accurately metered into the process at a wide range of powder feed rates.
The prior art discloses a variety of apparatuses for mechanically metering powder. For example, auger-type powder feeders are used commonly in industry. These feeders use a rotating screw or turbine wheel to move powder from a hopper into the process destination. With these feeders, often the rotation of the screw results in periodic pulsations of powder as the screw turns. Additionally, due to the abrasive nature of metallic powders, high wear typically occurs on the screw and drive components.
One solution to the problems associated with auger-type powder feeder is the vibratory feeder. For example, U.S. Pat. No. 2,187,717 (Weyandt) describes a vibratory electrical apparatus for conveying or handling difficult to move or handle materials, such as pastes, powders, sand, gravel, packages, articles and the like. U.S. Pat. No. 3,788,449 (Baberowski, et al) discloses a vibratory conveyor capable of periodic rotational and vertical vibrations.
Using vibrating tables to effect forward movement of powder particles is known in the additive manufacturing industry. With these devices vibration forces are imparted to powder particles situated on a table. The vibrating particles then fall off the edge of the vibrating table and into the process destination. This is a very inexpensive, simple, and robust method. However, the pulsations generated by the vibration results in perturbations in the powder flow, which is unacceptable for many additive manufacturing applications.
The prior art also discloses a variety of apparatuses that meter powder via a combination of mechanical moving elements and a motive gas. U.S. Pat. No. 3,909,068 (Coucher) and U.S. Pat. No. 3,517,861 (De La Vega) disclose examples of such devices. U.S. Pat. No. 4,227,835 (Nussbaum) discloses an apparatus that feeds powder using a continuous annular groove on a rotating metering plate and then sucks the powder out of the groove after the plate has rotated through a certain angle. Powder metering devices like those of Nussbaum can give smooth and accurate powder flow, even at relatively low flow rates. However, the technique of Nussbaum requires significant gas flow alongside the powder in order to effect the suction force required to evacuate the groove. This can be a problem for applications that require low gas flow or zero gas flow. Moreover, rotary disc feeders involve many moving parts and seals, which parts and seals increase the complexity and ultimately the cost of the powder feeder.
The aforementioned prior art devices for mechanically metering powder generally work well at high mass flow rates. However, these techniques become inconsistent at lower flow rates (under 1 gram per minute). Additionally, prior art devices that meter powder mechanically are often physically large and bulky and take up valuable space in the equipment or facility into which they are integrated. Existing mechanical metering powder feeders also involve significant complexity in their design and construction, which ultimately leads to high manufacturing and production costs.
Another deficit in the mechanical powder feeder of the prior art is that it must be expensively adapted in order to operate under conditions of high pressure, vacuum or high temperature. Such adaptations are exemplarily shown in U.S. Pat. No. 6,651,843 (Kowalsky, et al), which describes a method of adapting a disc powder feeder for high-pressure applications by enclosing the entire device in a pressure vessel to reduce the strain on the dynamic seals of the device.
Devices for metering powder without use of moving mechanical parts have also been suggested. For example, U.S. Pat. No. 4,071,169 (Dunn) describes a powder metering device that achieves very low flow rates utilizing electrostatically induced oscillation. U.S. patent application Ser. No. 14/994,973 also describes an electrostatic powder feeder. The entirety of that application is incorporated herein by reference. With the device of this application, a DC voltage supply creates an electric potential between first and second electrodes, which in a preferred embodiment are spaced 5-10 mm apart. A preferable potential for this spacing will be between one thousand and ten thousand volts between the first and second electrodes. The hopper gravity feeds (drops) an amount of powder on the first electrode. The powder upon the first electrode, being somewhat conductive, develops an electric charge on its surface due to the potential difference between the electrodes. However, by virtue of the electric potential and the positioning of the second electrode relative to the first electrode, the powder particles with the surface charge on the first electrode are caused to move (jump) initially off the first electrode toward the second electrode. The second electrode is preferably above the first electrode, wholly or partially to the side of the first electrode or both. In a direct mode feeder, the powder particles are caused to move toward the second electrode drop away from the second electrode due to the force of gravity. Preferably, the drawn particles will drop away before reaching the second electrode. Though even if they do reach the second electrode, they will eventually drop out of the feeder due to the force of gravity without ever re-contacting the first electrode or the powder pile upon it. U.S. patent application Ser. No. 14/994,973 also discloses a stochastic mode electrostatic feeder. With this type of feeder the particles oscillate between the electrodes in a manner that imbues the particles with two or three dimensions of motion (as opposed to just one dimension of travel to and from electrodes) such that a percentage of the oscillating particles will eventually move to a space in the feeder wherein the particles are no longer constrained between the electrodes and the force of gravity will allow them to escape the electrode space and be dispensed by the feeder.
The prior art electrostatic powder metering devices do, indeed, result in consistent powder flow at low flow rates. However, they require the presence of open, uninsulated, high-voltage electrodes in the presence of fine metallic powders. This situation can result in electrical arcing between the electrodes. When it occurs, this arcing causes powder feeding to stop until the arc is extinguished. The arcing can even result in the dangerous igniting of the powders.
In view of the deficits of the prior art powder metering devices, an improved powder metering device is desirable. In particular, it would be desirable to provide a device and method of metering powder particles that operate well at low mass flow rates (under 1 gram per minute) as well as higher feed rates. It is further desirable that metering devices and methods be able to quickly change between flow rates and be less complex and expensive to produce than prior art devices and methods. In addition, the devices and methods should provide these benefits while also providing highly consistent powder flow for additive manufacturing, welding, and cladding.